A couple of things… you said he was suppose to be watching the kids. Were they awake? or young enough to be taking naps or something where he was safely hidden from them? If not or if what he was looking at were some kind of deviant (like pedo) images then you have a big problem and can skip the rest of the post. If not and it was just straight guy / girl stuff, read on…
Secondly how is your marriage? Is he attracted to you? What I’m getting at is this behavior, which is a mortal sin, a replacement for you because he isn’t attracted or is attracted, big difference. You two need to discuss that.
Thirdly, looking at porn isn’t the end of the world. There are a LOT of mortal sins which people do and everyone seems to overlook. Gossip can be a mortal sin? How many women sit around an gossip during the day… possibly even on this board? Yet we seem to only have threads like this when it’s “porn.”
The fact is that we have a sex charged society. Many men, even the most Christian men I know, fail at this from time to time. I do not know a single one of them that would consider replacing his wife with a fantasy. Most of these men aren’t the ones to physically cheat either. And truly most of the women I know don’t care… yup they don’t care. They do not take that verse in a literalist sense… I’m not sure we do as Catholics either. At least I don’t know anyone who dose except for the above poster… perhaps we should ask Fr. Serpa but I don’t see porn as adultery. There is no physical contact.
Truly, until I started posting here a year ago I only knew one woman, out of 37 years of life (and I know a lot of people), who got offended when she found out her husband had viewed porn. Many of these women are Christians too. No some are not, those who aren’t have their own porn collection… and I come from the conservative south mind you. When I say view I mean from time to time, not a $300 a month daily subscription or anything. There is no repentance when you have a 12 month contract…
Now all that being said, let me go back to the point. Why is he doing this? Is it because he thinks of you or someone else? That is the real question. Morally he is objectifying women and that’s wrong… He needs to talk with you and go to confession. But if he hasn’t looked at porn in 8 months, he isn’t addicted, he is just bored.

Talk about it… as adults, not like it’s the end of the world or your marriage. You can figure this out together.
Joe
PS - Just FYI though, he isn’t perfect and he will fail again, if not at this, at something else. Remember that a Sacramental Marriage isn’t 50/50… it’s 100/100. As humans we don’t always live up to that, focus on each other and TALK!